The present invention relates to a computer circuit for selectively obtaining automatic dynamic compression or expansion of an input signal.
Companders are used to improve the signal-to-noise ratio in the transmission of signals over transmission paths as well as for the storage of signals. In operation, the magnitude of relatively weak useful signals is amplified prior to transmission or storage more than that of relatively large magnitude signals and then lowered after transmission or playback. The noise signals, which are superimposed on the useful signals during transmission, are also lowered after transmission and can be suppressed to some degree by the useful signals. The suppression effect is a physiological phenomenon and depends on the amount of frequency deviation between the useful signal and the noise signal. It is therefore advisable to divide the low frequency spectrum into a plurality of frequency selective channels and to control the effective transmission factor in these channels as a function of only the useful signals being transmitted.
Complementary behavior for both compression and expansion can be obtained in a compander by using the same adjusting devices for both modes of operation. Specifically, the adjusting devices are connected in the feedback path of an amplifier for the compression mode of operation and in the signal path including the amplifier for the expansion mode of operation.
It has been found that in the operating mode employing feedback there is a tendency toward oscillation which is determined mainly by the transmission characteristics of the highest frequency channel. This tendency to oscillate occurs because the lowpass filters in the feedback path formed of the circuit and transistor capacitances produce a delay in the signals and therefore a shift in phase. If, for example, an amplifier is located in the feedback path so as to compensate for the high attenuation caused by the adjusting device, the critical phase shift required for resonance may be reached in the high frequency channel.
It is therefore an object of the present invention to provide a feedback path for the high frequency channel of a circuit for automatic dynamic compression or expansion which exhibits a short signal delay. Another object is to provide a circuit for automatic dynamic compression or expansion which does not require a differential amplifier in the high frequency channel.